nyscience - 5. Energy and matter interact

MAJOR UNDERSTANDINGS
5.1a The motion of an object is always judged with respect to some other object or point. The idea of absolute motion or rest is misleading.
5.1b The motion of an object can be described by its position, direction of motion, and speed.
5.1c An object's motion is the result of the combined effect of all forces acting on the object.
5.1d Force is directly related to an object's mass and acceleration. The greater the force, the greater the change in motion.
5.1e For every action there is an equal and opposite reaction.
5.2a Every object exerts gravitational force on every other object. Gravitational force depends on how much mass the objects have and on how far apart they are. Gravity is one of the forces acting on orbiting objects and projectiles.
5.2b Electric currents and magnets can exert a force on each other.
5.2c Machines transfer mechanical energy from one object to another.
5.2d Friction is a force that opposes motion.
5.2e A machine can be made more efficient by reducing friction. Some common ways of reducing friction include lubricating or waxing surfaces.
5.2f Machines can change the direction or amount of force, or the distance or speed of force required to do work.
5.2g Simple machines include a lever, a pulley, a wheel and axle, and an inclined plane. A complex machine uses a combination of interacting simple machines, e.g., a bicycle.